Generation of boron-trifluoride and sulphuric acid from boron trifluoride hydrate

ABSTRACT

The invention relates to the preparation of BF 3  and H 2 SO 4  of commercial grade from boron trifluoride hydrate effluents containing organic impurities.  
     The process consists essentially in reacting the said effluent with oleum, in recovering the gaseous boron trifluoride thereby liberated and in subjecting the sulphuric acid by-product to treatment with hydrogen peroxide and to purging with air.

DESCRIPTION

[0001] The present invention relates to the field of boron trifluoride and sulphuric acid and relates more particularly to the conversion of industrial effluents of boron trifluoride hydrate to boron trifluoride and sulphuric acid.

[0002] Boron trifluoride is a gas which is principally used in industry as a catalyst in a large number of reactions: polymerization, esterification, alkylation and isomerization. Customarily, after use, the boron trifluoride is treated with an aqueous sodium hydroxide solution and the resulting effluent, made up of derivatives containing fluorine and borine, is discarded.

[0003] In order to avoid these fluorine- and borine-containing wastes, the boron trifluoride can be recovered after use, at the end of reaction, in the form of solutions of BF₃ hydrate which are obtained on the one hand by bringing the BF₃ into contact with water and, on the other hand, by washing the organic compounds formed in the reaction catalysed by BF₃ with water (see, for example, the patent EP 364 815).

[0004] Owing to the presence of—essentially organic—impurities, the solutions of boron trifluoride hydrate are generally coloured to a greater or lesser extent, it being possible for their content of organic carbon to range from several ppm to several thousand ppm (customary values: from approximately 10 to 1000 ppm).

[0005] The patent U.S. Pat. No. 5,536,484 describes a process for upgrading technical BF₃ hydrates in the form of aqueous solutions of tetrafluoroboric acid corresponding to commercial specifications. However, the final step of decolouring requires the use of an active carbon which must be destroyed after use.

[0006] Another means of upgrading a technical BF₃ hydrate is set out in the patent EP 364 815 and consists of regenerating the boron trifluoride by treating the technical hydrate with sulphuric acid, oleum or SO₃. Unfortunately, the sulphuric acid resulting from this operation has a yellow to black coloration, making it unsuitable for use without prior purification treatment.

[0007] On the other hand, organic substances and carbon dioxide are unacceptable in a commercial BF₃ intended for applications in catalysis. The current commercial product must contain less than 10 mg of organic carbon and less than 5 mg of CO₂ per kg of BF₃.

[0008] A process has now been found which enables a boron trifluoride and a sulphuric acid both of which correspond to commercial specifications to be prepared from technical BF₃ hydrate and oleum. This process is particularly advantageous in that it produces no waste and does not require the use of active carbon.

[0009] The process according to the invention comprises in succession the following steps:

[0010] (a) reacting oleum with the technical BF₃ hydrate,

[0011] (b) recovering the gaseous BF₃ thereby liberated, and

[0012] (c) subjecting the sulphuric acid by-product of step (a) to treatment with hydrogen peroxide and to treatment with air.

[0013] BF₃ hydrate is a dihydrate of formula BF₃.2H₂O and is employed in accordance with the process of the invention in the form of an aqueous composition referred to as technical BF₃ hydrate. This composition essentially comprises a mixture of water and dihydrate along with the abovementioned impurities. It is generally obtained by absorption of the gaseous BF₃ in water following its use in various processes of polymerization (poly-α-olefins, petroleum resins, etc.) and by washing the polymer obtained from this polymerization with water. It may also originate from the hydrolysis of used BF₃ complexes such as, for example, BF₃.O(C₂H₅)₂.

[0014] The BF₃ content of the technical BF₃ hydrate to be treated may vary within wide limits, but for greater ease of upgrading it is preferred to use a technical BF₃ hydrate having a concentration of between 35 and 65.3%, preferably between 47 and 65.3% (expressed in weight of BF₃ per 100 g of aqueous composition). Unless specified otherwise, the percentages indicated in the present text are percentages by weight.

[0015] The BF₃ content of the technical BF₃ hydrate may also be expressed by the number x of mols of water, corresponding to molecules of free—i.e. uncomplexed—water, per mole of BF₃. The above-defined ranges of content by weight for BF₃ therefore correspond to a value of x of between approximately 0 and 5 for the broad range and between approximately 0 and 2.2 for the preferred range. The mixture of water and dihydrate present in the technical BF₃ hydrate may therefore be defined by the term BF₃.2H₂O+xH₂O.

[0016] If required, one means of attaining this BF₃ content consists, as described in the patent EP 364 815, in concentrating a dilute BF₃ hydrate by removal of water under vacuum; the concentrated hydrate drawn off at the bottom of the column contains traces of heavy organic substances (content of organic carbon: from approximately 10 to 1000 ppm).

[0017] The oleum employed in the process of the invention is a solution of sulphuric anhydride, SO₃, in sulphuric acid, the SO₃ content of the said solution being between 5 and 65% and, preferably, between 10 and 65%. This content may also be expressed by the number y of moles of H₂SO₄ which solubilize 1 mol of SO₃. The ranges of SO₃ content by weight indicated above therefore correspond to a value of y of between 0.44 and 15.5 and, preferably, between 0.44 and 7.4. The oleum can be defined by the expression: SO₃+yH₂SO₄.

[0018] The preparation of boron trifluoride from the technical BF₃ hydrate and oleum (step a) corresponds to the following reaction:

(BF₃.2H₂O+xH₂O)+(2+x)[(SO₃ +yH₂SO₄]->BF₃+(2+x+2y+xy)H₂SO₄  (1)

[0019] in which x and y are as defined above.

[0020] The amounts of oleum and technical BF₃ hydrate reacted in step (a) of the process of the invention are advantageously chosen such that the amount of oleum (expressed in numbers of mol of SO₃) divided by the amount of technical hydrate (expressed in total number of mols of free or complex water) is between 0.5 and 1.5 and, preferably, is close to 1.

[0021] The reaction of step (a) is generally carried out at a temperature of between 75 and 110° C., preferably between 100 and 110° C.

[0022] The gaseous boron trifluoride liberated by the reaction and recovered, in accordance with step (b) of the process of the invention, generally at the top of the reactor, contains neither organic substances nor inert substances such as nitrogen or carbon dioxide. It has all of the characteristics of a commercial product and can be processed for supply in the usual fashion, known per se—by compression, for example.

[0023] In accordance with step (c) of the process of the invention, the sulphuric acid produced in step (a) is subjected to treatment with hydrogen peroxide and to treatment with air, the said treatments being carried out in succession and in either order.

[0024] The treatment with H₂O₂ ensures oxidative destruction of the organic compounds present as impurities in the sulphuric acid produced in step (a), these impurities originating from the technical BF₃ hydrate. The chemical reaction employed is:

C_(organic)+2H₂O₂->CO₂+2H₂O  (2)

[0025] in which C_(organic) represents the said organic impurities, also referred to as “carbon of organic origin”.

[0026] This treatment H₂O₂ advantageously produces a colourless H₂SO₄.

[0027] The amount of hydrogen peroxide to be used can vary within wide limits. For an economic treatment, this amount is advantageously determined as follows: the weight of carbon of organic origin present in the technical BF₃ hydrate employed in step (a) is determined quantitatively using a total organic carbon analyzer. The amount of H₂O₂, expressed in numbers of moles, is between 4 and 200 times the number of molar equivalents of carbon of organic origin determined in this way, preferably between 5 and 20 times.

[0028] The hydrogen peroxide is generally employed in the form of an aqueous solution whose concentration is between 3 and 70%, preferably between 10 and 70%.

[0029] Treatment with H₂O₂ is carried out at a temperature of between 80 and 115° C., preferably between 105 and 110° C.

[0030] The air treatment of step (c) of the process of the invention makes it possible to remove essentially all of the boron trifluoride dissolved in the sulphuric acid, the preferred objective being to reduce its BF₃ content to a value of less than 50 ppm. This treatment is generally carried out by purging. The BF₃ thereby released can advantageously be absorbed in water so as to give a BF₃ hydrate which can optionally be recycled to step (a) of the process of the invention.

[0031] The process of the invention can be operated continuously or batchwise.

[0032] If operated batchwise, step (a) of the process of the invention is carried out in a first reactor. The treatment with H₂O₂ and the air purging can be carried out in two different reactors or in the same reactor, the said reactor or reactors optionally being that used in step (a). It is possible either to carry out the treatment with H₂O₂ first and then the air purging, or vice versa.

[0033] When the process is, preferably, operated continuously it is advantageous to use three reactors in series, the first for carrying out the reaction of the oleum with the technical BF₃ hydrate and recovering the gaseous BF₃ at the top, and the two others for carrying out, in succession but in either order, the treatment with H₂O₂ and the air purging.

[0034] Preferably, the first reactor is a stirred reactor in which the level of the reaction medium (H₂SO₄ 100%) is kept constant by means of an overflow leading into the second reactor.

[0035] The examples which follow illustrate the invention without limiting it. Unless specified otherwise, the percentages are by weight.

EXAMPLE 1

[0036] An installation is used which comprises two reactors in series. The first reactor has a capacity of 400 ml (200 ml used for volume for the reaction medium) and is equipped with a (4-blade) helical stirrer with a diameter of 4 cm; the rotary speed of the stirrer head is 500 revolutions per minute. This reactor is equipped with a jacket traversed by a heat transfer fluid in order to dissipate the heat of reaction and maintain the temperature at between 104 and 107° C.

[0037] The two reactants, technical BF₃ hydrate and 65% oleum, are weighed out continuously and introduced into the reactor, which contains an initial charge of 200 ml of 100% sulphuric acid. The technical BF₃ hydrate, supplied by means of a peristaltic pump, and the oleum, supplied via a piston-type metering pump, arrive in the reaction medium via two dip tubes arranged side by side. The overflow of the reactor is directed towards a second reactor having the same characteristics as the first (volume, stirring, etc.). The technical BF₃ hydrate has the following characteristics: BF₃ = 55.3% H₂O = 44.7% density =  1.505

[0038] and its content of organic carbon is 83 mg per kg of technical BF₃ hydrate.

[0039] The oleum used assays at 65% SO₃ and 35% H₂SO₄. The feed rate of technical BF₃ hydrate is 214 g/h and that of oleum is 669 g/h, corresponding to a ratio of (number of mols of SO₃ in the oleum) to (number of mols of free or complexed water of the BF₃ hydrate) of 1. The flow rate of sulphuric acid at the overflow is 773 g/h.

[0040] The sulphuric acid emerging from the first reactor is treated in the second reactor with hydrogen peroxide; the amount of H₂O₂ employed is 3.1 g/h of 10% H₂O₂ and the temperature of the reaction medium is maintained-at between 104 and 107° C.

[0041] In order to recover the dissolved BF₃ (1.5% by mass) in the sulphuric acid from the second rector this acid is subjected to air purging. The BF₃ content of the treated sulphuric acid is less than 50 ppm and the BF₃ present in the purging air is absorbed in water in a column and recycled in the form of an aqueous BF₃ solution.

[0042] After the BF₃ has been purged with air, the sulphuric acid assays at 99.9% H₂SO₄, and this sulphuric acid, emerging from the second reactor by way of an overflow, is cooled to room temperature.

[0043] This installation operated continuously for 7 hours. The BF₃ liberated was recovered at the top of the first reactor in a water trap (capacity 5 litres) stirred by means of a magnetic bar. The BF₃ trapped in the water was analyzed to determine its contents of organic carbon and of inorganic carbon (CO₂ gas). More than 90% of the BF₃, initially in the form of BF₃ hydrate, was recovered in the form of BF₃ gas, which contains less than 5 mg of organic carbon/kg of BF₃ gas. No trace of carbon dioxide was detected (detection limit: 1 mg of CO₂/kg of BF₃ gas). This BF₃ has the characteristics of commercial BF₃ gas.

[0044] The sulphuric acid obtained contains less than 10 mg of organic carbon/kg (detection limit). This grade of sulphuric acid is colourless and is therefore easy to market.

EXAMPLE 2 (COMPARATIVE)

[0045] The same process is applied as for Example 1 but without introducing hydrogen peroxide into the second reactor. The BF₃ recovered in the first reactor has the characteristics of a commercial BF₃ (content of organic carbon of less than 5 mg/kg of BF₃, no trace of CO₂ detected) but the sulphuric acid emerging from the second reactor is black and contains 20 mg of organic carbon/kg of H₂SO₄.

EXAMPLE 3: (COMPARATIVE)

[0046] The process of Example 2 is reproduced using a technical BF₃ hydrate having the following characteristics: BF₃ = 47.7% H₂O = 52.3% density =  1.38

[0047] content of organic carbon: 620 mg/kg of technical BF₃ hydrate.

[0048] The feed rate of the technical BF₃ hydrate is 188 g/h and that of the oleum is 681 g/h. The amount of residual sulphuric acid from the overflow of the second reactor is 784 g/h. Before purging of the sulphuric acid, the amount by mass of BF₃ is 1.3%. After air purging of the BF₃, the sulphuric acid assays at 98.4% H₂SO₄ and 1.6% H₂O.

[0049] The BF₃ liberated is recovered at the top of the first reactor in a water trap and the BF₃ trapped in the water is analysed in order to determine its contents of organic carbon and of inorganic carbon (CO₂ gas); it contains less than 10 mg of organic carbon per kg of BF₃.

[0050] The sulphuric acid recovered at the overflow of the second reactor contains 155 mg of organic carbon per kg of sulphuric acid. It has a dark-brown colour and is therefore unmarketable.

EXAMPLE 4: (COMPARATIVE)

[0051] A technical BF₃ hydrate is used which assays at 53.5% BF₃ (d=1.47) and contains 780 mg of organic carbon/kg of BF₃ hydrate.

[0052] The feed rate of the BF₃ hydrate in the reactor is 210 g/h and that of the 65% oleum is 667 g/h. 70% hydrogen peroxide is introduced at a rate of 4.1 g/h into this same reactor.

[0053] At the overflow of the reactor, the flow of sulphuric acid is 784 g/h. This acid contains 1.1% of BF₃. After purging, the sulphuric acid assays at 97.2% H₂SO₄ and 2.8% of water.

[0054] The BF₃ emerging at the top of the reactor contains 5300 mg of CO₂ per kg of BF₃, making it unsuitable for commercial use. The content of organic carbon in the sulphuric acid is less than 10 mg per kg of acid.

EXAMPLE 5

[0055] The operating technique employed is the same as that described in Example 4 except that the hydrogen peroxide is introduced into the second reactor, with the same rate of 4.1 g/h of 70% H₂O₂.

[0056] The BF₃ emerging at the top of the first reactor has the characteristics of a commercial BF₃ (content of organic carbon of less than 5 mg/kg of BF₃, no trace of CO₂ detected). The sulphuric acid emerging from the second reactor is colourless, contains less than 10 mg of organic carbon per kg of acid, and can therefore be used commercially.

[0057] Although the invention has been described in conjunction with specific embodiments, it is evident that many alternatives and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, the invention is intended to embrace all of the alternatives and variations that fall within the spirit and scope of the appended claims. The above references are hereby incorporated by reference. 

1. Process for preparing boron trifluoride and sulphuric acid from a technical BF₃ hydrate and oleum, characterized in that it comprises in succession the following steps: (a) reacting the oleum with the technical BF₃ hydrate, (b) recovering the gaseous BF₃ thereby liberated, and (c) subjecting the sulphuric acid by-product from step (a) to a treatment with hydrogen peroxide and to a treatment with air.
 2. Process according to claim 1, characterized in that the BF₃ content of the technical BF₃ hydrate is between 35 and 65.3%, preferably between 47 and 65.3%.
 3. Process according to either of claims 1 and 2, characterized in that in step (a) the amount of oleum (expressed in numbers of moles of SO₃) divided by the amount-of technical hydrate (expressed in total number of moles of free or complexed water) is between 0.5 and 1.5 and, preferably is close to
 1. 4. Process according to one of claims 1 to 3, characterized in that step (a) is carried out at a temperature of between 75 and 110° C., preferably between 100 and 110° C.
 5. Process according to one of claims 1 to 4, characterized in that the amount of H₂O₂, expressed in numbers of moles, is between 4 and 200 times the number of molar equivalents of carbon of organic origin present in the technical BF₃ hydrate, preferably between 5 and 20 times.
 6. Process according to one of claims 1 to 5, characterized in that the treatment with H₂O₂ is carried out at a temperature of between 80 and 115° C., preferably between 105 and 110° C.
 7. Process according to one of claims 1 to 6, characterized in that it is operated continuously and the reactor in which the reaction of the oleum with the technical BF₃ hydrate takes place is a stirred reactor in which the level of the reaction medium is kept constant by means of an overflow leading into a second reactor. 